Using a conventional clutch mechanism which disconnects the gearbox's input shaft from the combustion engine during a shifting process in the gearbox entails disadvantages, such as heating of the clutch mechanism's discs, which results in an increased fuel consumption and wear of the clutch discs. There are also large losses as a result, in particular when the vehicle is started. A conventional clutch mechanism is also relatively heavy and costly. It also occupies a relatively large space in the vehicle. Friction losses also arise at the use of a hydraulic converter/torque converter commonly used in automatic transmission. By ensuring that the vehicle has a drive system in which the output shaft of the combustion engine, the rotor of the electrical machine and the input shaft of the gearbox are connected with a planetary gear, the conventional clutch mechanism and the disadvantages associated therewith may be avoided. A vehicle with a drive system of this type constitutes prior art as set out in EP 1 319 546 and SE 536 329.
Although this drive system, especially the one described in SE 536 329, functions well and has a range of advantageous features, efforts are constantly made to improve such a drive system with respect to its behavior and function in certain operating situations.